Thesis
On the characterisation of structural damping in wind turbine blade composite materials : an approach using dynamic mechanical analysis and experimental modal analysis
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2025
- Thesis identifier
- T17377
- Person Identifier (Local)
- 201981662
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- In wind turbine blade design, carbon fibre composites are increasingly employed in larger, more modern blades for their superior strength and stiffness; however, their inherent anisotropic nature under dynamic loads leads to complex and poorly understood damping mechanisms. Despite the advantages of these materials, current experimental approaches have failed to capture the directional variations in damping behaviour. This has led to an identifiable knowledge gap, which has resulted in non-optimised damping performance for wind turbine blades. This thesis addresses these shortfalls by developing two complementary experimental methodologies, which enable a more detailed damping characterisation. The first was an integrated approach that used a new FEA assisted DMA framework, combining Dynamic Mechanical Analysis (DMA) with Finite Element Analysis (FEA) in order to quantify strain energy distributions. The second employed Experimental Modal Analysis (EMA) and a developed experimental test rig to characterise the damping properties of carbon fibre composite samples. This research initially employed a FEA-assisted DMA methodology, integrating FEA with DMA by incorporating experimental DMA data and FEA strain results of the composite specimens into a new model. Traditional DMA methods historically have only provided an overall damping measure and did not quantify the anisotropic contributions inherent in carbon fibre composites, necessitating the hybrid FEA approach. This integration enabled strain energy results to be extracted using an ANSYS model that replicated the experimental setup, thereby allowing the decomposition of bulk damping properties into directional components. The directional damping quantities obtained from the model then enabled the complex material behaviour to be predicted using existing models. Extensive DMA testing of unidirectional (fibre aligned) and transverse samples, of 60 mm length (50 mm span) across varying thicknesses, validated the expected damping mechanisms associated with material anisotropy. The findings demonstrated that damping behaviours in the fibre-aligned direction differed significantly from those in the transverse direction, with shear contributions playing a vital role in energy dissipation. Additionally, the damping component measured in the fibre direction was observed to be approximately 10% lower than that of the thinnest samples (2mm), with similar trends evident in the transverse results, further corroborating the influence of shear effects. Although this thesis primarily focused on damping in the fibre direction, which was the principal loading direction for the carbon fibre in the spar caps of turbine blades, the analysis also highlighted the importance of considering damping in other orientations, which enabled a more complete understanding of the composite’s dynamic response. In parallel, EMA was undertaken as a complementary technique to DMA, as the two methods captured different aspects of damping: DMA provided a stress–strain based measure of damping properties, in the form of tan(δ), whereas EMA delivered a vibrational-based, bulk damping ratio ζ. In contrast to DMA, which provides damping at the material level using shorter specimens, where contributions from non-fibre directions are much more pronounced, EMA testing involved geometries with a much larger aspect ratio. This structural-level configuration made off-axis damping contributions negligible, thereby simplifying the isolation of fibre-directional behaviour. DMA offered detailed insights into local damping properties and strain energy distribution, whereas EMA provided a complementary structural perspective by capturing the modal responses of the specimens. EMA testing was performed on larger samples (up to 2 m in length) using a custom designed rig that incorporated a vacuum chamber with automated sample excitation and nodal suspension with limited environmental control. The setup included an automated pneumatic impact hammer, accelerometers, strain guages and a calibrated National Instruments data acquisition system, which provided both strain and acceleration data. This was then processed to yield mode shapes, modal frequencies, and damping ratio values. The rig was specifically designed to minimise external energy losses, seen in conventional EMA applications, that could be inadvertently recorded as damping in traditional EMA methodologies. Following the experimental investigations, the DMA and EMA results were compared using an analytical model designed to convert one form of damping measurement into the other. However, the implementation of this model revealed that the correlation between the two sets of results was weaker than had been anticipated. This discrepancy was mainly attributed to differences in environmental conditions and the fact that DMA testing was not performed at the specimens’ first natural frequency due to the limitations of the equipment used. Consequently, while confidence was maintained in the reliability of both methodologies, it was concluded that a direct conversion between these two distinct damping quantities was not feasible using standard analytical conversion methodologies. By integrating computational modelling with experimental testing, this research characterised the directional damping mechanisms in carbon fibre composites used in wind turbine blades. The novel FEA-assisted DMA approach quantified strain energy distributions and directional damping responses, clearly demonstrating anisotropy and the critical role of shear effects. EMA complemented these findings by providing structural-level damping measurements. Together, these methodologies provide actionable pathways by enabling targeted design and material selection specifically to enhance damping performance. Improved damping reduces vibrational amplitudes, leading to lower structural fatigue and thus extending blade lifespan. Additionally, the detailed directional damping characterisation informs more accurate numerical models, increasing the reliability of simulations used in blade design processes. Ultimately, this supports the development of optimised blades that are not only lighter, thereby reducing material usage and costs, but also more resilient, leading to improved structural performance, increased energy output efficiency, reduced maintenance frequency, and greater overall sustainability in wind turbine technology.
- Advisor / supervisor
- Nash, David H.
- Kazemi-Amiri, Abbas Mehrad
- Resource Type
- DOI
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PDF of thesis T17377 | 2025-06-17 | Public | Download |